Advances in the field of digital magnetic recording have led to an increase in recording densities of storage media, such as magnetic disks. The higher overall densities can be attributed to increases in both linear and track densities. Specifically, each of the recorded signals occupies a smaller are of the medium and thus has a smaller energy content in the form of magnetic flux. Accordingly, thin-film read/write heads of reduced dimensions and higher sensitivity are needed to offset the weaker signals retrievable from the storage media during read operations.
The performance of a thin-film head depends upon the magnetic properties of its materials and the geometry of the head structure. A conventional thin-film magnetic head consists of two poles formed of "soft" magnetic material which is easily magnetized and has a relatively high permeability. The poles are joined at one end, called the yoke, and are separated by a gap layer that is precisely defined at the other end, called the tip. The turns of a magnetic coil are embedded in the gap layer adjacent the yoke. During read and write operations, the head is positioned with the tip portion adjacent the disk; the coil provides a mechanism for driving magnetic flux into as well as sensing magnetic flux from the head.
In order to facilitate magnetic flux conduction between the poles and coil, the yoke region is made as wide as possible; that is, the poles are generally heart-shaped and superimposed upon one another in a symmetrical configuration. Such a large surface area minimizes the yoke resistance and increases the available amplitude of the signal emanating from the head; however, it also increases inductance and flux leakage, i.e. flux in the head that does not link the coil.
Thin-film head materials are composed of individual regions or domains with local magnetizations equal to the saturation magnetization of the material. The boundaries between these regions are called domain walls. When a head is writing or reading data, the rotation of magnetization within these domains, i.e. "conduction by rotation", or the shift of domain walls, i.e. "domain wall motion", constitutes the head response to magnetic fields.
Thin-film heads are manufactured by processes such as electroplating and vacuum deposition through a succession of masks onto a wafer, and the wafer is then diced into individual units. During deposition of the magnetic material, uniaxial anisotropy is induced by the imposition of a strong magnetic field. The field creates a magnetic anisotropy in the material that results in a domain pattern having a rest state of magnetization in particular directions, i.e. the "easy axes". When an easy axis is transverse to the direction of flux conduction, the flow of flux through the head can occur by small-angle rotations of the rest state of magnetization away from the easy axis. In other words, by inducing domain patterns having a series of easy axes of magnetization transverse to the direction desired to propagate flux, the magnetic flux can conduct by the process of small-angle rotations of these axes.
Magnetic flux can also be conducted through a ferromagnetic material by domain wall motion. However, this mode of conduction is characterized by a relatively low permeability at low field levels. This mode of conduction is also ineffective at high frequencies because of the long time delay associated with the shifting of domain walls and the loss of permeability due to skin effect. Domain wall motion is therefore much less suitable than axis rotation for the sensing of weak, high-frequency fields associated with high-density disks.
Moreover, conduction by domain wall motion is also more susceptible than axis rotation to domain structure imperfections, e.g. inhomogeneity of the thin-film materials, during read operations. When magnetic flux encounters a defect in a domain wall during conduction, signal processing distortion results. If the drive field is insufficient to move the wall past the defect, e.g. pinning site, the wall will stick permanently and read-back will cease. Otherwise, the domain wall will stick temporarily and flux conduction will pause with a corresponding loss of output voltage. The wall subsequently snaps free and a spurious voltage spike occurs, resulting in Barkhausen noise.
Further discussions of flux conduction in thin-film heads and comparison of conduction by rotation and domain wall motion are contained in the following papers, which are expressly incorporated by reference as though fully set forth herein:
Mallary, M., et al. "Three Dimensional Transmission Line Model for Flux Conduction in Thin-Film Recording Heads", Journal of Applied Physics, Vol 67, pp 4863-4865 (1990);
Mallary, M., et al. "Frequency Response of Thin Film Heads with Longitudinal and Transverse Anisotropy", IEEE Transactions on Magnetics, Vol 26, pp 1334-1336 (1990);
Mallary, M., "Conduction of Flux at High Frequencies in Permalloy Strips by Small-Angle Rotations", Journal of Applied Physics, Vol 57, pp 3952-3954 (1985); and
Mallary, M., and Smith, A. B., "Conduction of Flux at High Frequencies by a Charge-Free Magnetization Distribution", IEEE Transactions on Magnetics, Vol MAG-24, p. 2374 (1988).
One approach to resolving the inductance and flux leakage issues associated with conventional thin-film head structures has been to arrange the poles of the magnetic head a certain distance from one another in a plane parallel to the surface of the poles. The poles overlap at the tip and at a portion of the yoke, referred to as the "flux via" region, but are laterally staggered elsewhere. This arrangement minimizes the overlapping area of the poles, which in turn reduces the flux leakage and inductance of the head. However, the magnetic flux must conduct in a horizontal or lateral direction by domain wall motion between the tip and the flux via.
Therefore, it is among the objects of this invention to provide an architecture that increases the efficiency of thin-film magnetic heads.
Another object of the present invention is to provide a thin-film head structure that increases the amplitude of signals processed during read operations.
Another object of the invention is to reduce domain wall motion induced noise (Barkhausen noise) in thin-film heads by minimizing the lateral conduction of magnetic flux.